If you use REBUS to analyze your nuclear reactors, and you use a lot of regions, you may be interested in page 13 of this pdf. You can change one line of the source code and the point where it says: “HMG4C CALLED AFTER NEUTRONICS TO UPDATE COMPXS” will fly by instead of sitting there for a very long time. This is a huge fix that ANL knows about and fixed internally but didn’t re-release to RSICC. I think it only affects REBUS-PC, and that REBUS-3 is OK. Enjoy!